Biofilm development in a hotspot of mixing between shallow and deep groundwater in a fractured aquifer: field evidence from joint flow, chemical and microbiological characterization

International audienceBiofilms play a major role in controlling the fluxes and reactivity of chemical species transported in hydrologicalsystems. Their development can have either positive impacts on groundwater quality(e.g. attenuation of contaminants under natural or stimulated conditions), or possible negative effects on subsurfaceoperations (e.g. bio-clogging of geothermal dipoles or artificial recharge systems). Micro-organisms require bothelectron donors and electron acceptors for cellular growth, proliferation and maintenance of their metabolicfunctions. The mechanisms controlling these reactions derive from the interactions occurring at the micro-scalethat depend onmineral compositions, the biota of subsurface environment, but also fluid mixing, whichdetermines the local concentrations of nutriments, electron donors and electron acceptors.Hence, mixing zones between oxygen and nutriment rich shallow groundwater and mineralized deep groundwaterare often considered as potential hotspots of microbial activity, although relatively few field data document flowdistributions, transport properties, chemical gradients and micro-organisms distributions across these mixinginterfaces.Here we investigate the origin of a localized biofilm development observed in the fractured granite aquiferat the Ploemeur observatory (H+ network hplus.ore.fr).This biofilm composed of ferro-oxidizing bacteria isobserved in an 130m deep artesian well. Borehole video logs show an important colonization of the well by thebiofilm in the shallower part (0 to 60m), while it is inexistent in the deeper part (60 to 130m). As flow is localizedin a few deep and shallow fractures, we presume thatthe spatial distribution of biofilm is controlled by mixing between shallow and deep groundwater. To verify thishypothesis we conducted a field campaign with joint characterization of the flow and chemical composition ofwater flowing from the different fractures, as well as the microbiological composition of the biofilm at differentdepth, using pyrosequencing techniques. We will discussin this presentation the results of this interdisciplinary dataset and their implications for the occurrence of hotspotsof microbiological activity in the subsurface